[zfleeman]

Closes #137.

-r used to take only WIDTHxHEIGHT, so shrinking a clip meant working out a width that keeps the aspect ratio, and doing it again after a crop. Now -r takes three forms:

-r	1920x1080 input	Notes
1280x720	1280x720	Same as before
720	1280x720	The width follows the height
50%	960x540	A percentage of the height, rounded to an even number

The new forms use ffmpeg's scale=-2:<height>. The -2 makes ffmpeg pick the width that keeps the aspect ratio, rounded to an even number, which x264 and x265 need. The scale runs after the crop, so with -x the height and the percentage apply to the cropped area.

Changes

• twopass.py: a new TwoPass._scale_size() turns -r into the scale filter's width and height. _check_video_size() and _apply_video_filters() both use it instead of splitting WIDTHxHEIGHT themselves. A Resolution in any other format (like 720p, 0, or 0%) now raises a clear ValueError instead of an int() error. An odd height like -r 721 raises the existing even-size error for x264 and x265. The aspect-ratio warning only applies to WIDTHxHEIGHT, since the other forms always keep the ratio.
• __main__.py: RESOLUTION_PATTERN and its error message accept the new forms.
• web.html: the Resolution field's browser-side check accepts them, rejects 0 and 0%, and shows the hint "e.g. 1280x720, 720, or 50%".
• README.md: the -r row lists all three forms, and the Detailed Example notes that -r 1080 or -r 75% would have done the same job.

Checked

Real x264 encodes of 5 seconds of a 1920x1080, 60 fps Call of Duty clip:

Flags	Output
-r 1280x720	1280x720
-r 720	1280x720
-r 50%	960x540
-r 33%	632x356
-x 240x0x1440x1080 -r 50%	720x540
-r 721	"x264 needs an even width and height, but the Resolution (-r) is 721."
-r 720p	"Invalid Resolution (-r) '720p'. Use width x height, a height, or a percentage of the height, ..."

Tests

• test_twopass.py: scale arguments for each form, a percentage with a crop, odd-height rounding for 33%, no aspect warning for a height, and errors for odd heights (x264/x265 only) and bad formats.
• test_main.py: the web form accepts 1280x720, 720, 50%, and 33.3%, and rejects 720p and 0%.